1. Field of the Invention
The present invention relates to bio-fuel compositions, uses thereof and methods for the manufacture of bio-fuel compositions. The present invention has particular application to the formation of blends of bio-fuels, such as bio-oil, bio-diesel and ethanol.
2. Related Art
Biomass pyrolysis is the thermal decomposition of biomass (e.g. plant material such as wood and wood bark) substantially in the absence of oxygen. Biomass is typically a mixture of hemicellulose, cellulose, lignin and small amounts of other organics. These components typically pyrolyse or degrade at different rates and at different temperatures and by different mechanisms and pathways.
One traditional example of biomass pyrolysis is the production of charcoal, where the main product of the pyrolysis is char. Alternative biomass pyrolysis techniques provide a product which, after cooling, includes a substantial proportion of liquid. This liquid is typically a dark brown liquid having a heating value that is around one half the heating value of conventional fuel oil. The liquid is typically referred to as bio-oil. In many circumstances, it is the bio-oil which is the most valuable product of the pyrolysis reaction, since bio-oil can be easily stored for later use, e.g. for heat and/or electricity generation. Bio-oil typically is a homogenous hydrophilic mixture of polar organics and water.
The rate and extent of decomposition of the components of biomass depends on the process parameters of the pyrolysis reactor, e.g. the rate of heating of the biomass, the mode of heating of the biomass and the residence time of the subsequent products. In turn, these process parameters may also have an effect on the subsequent behaviour of the product, e.g. by secondary reactions such as cracking (of higher molecular mass products) or condensation reactions (of lower molecular mass products).
Biomass pyrolysis can be carried out using fast heating rates and short hot vapour residence times. Such “fast” pyrolysis processes are reviewed by Bridgwater et al (A. V. Bridgwater, D. Meier and D. Radlein, “An overview of fast pyrolysis of biomass” Organic Geochemistry Volume 30, Issue 12, December 1999, Pages 1479-1493). It is considered in that disclosure that optimum levels of organics in the bio-oil may be achieved by fast heating of the biomass to a reaction temperature of around 500° C. and hot vapour residence times of less than around 1 second.
There are several different options for achieving heating of the biomass in a fast pyrolysis reactor. For example, ablative pyrolysis requires the biomass particles to be pressed against a heated surface and rapidly moved. This allows the use of relatively large biomass particles. Alternatively, fluid bed and circulating fluid bed pyrolysis reactors transfer heat from a heat source to the biomass particles by a mixture of convection and conduction. Since heat transfer must typically occur quickly, fluid bed pyrolysis reactor require the use of small biomass particles, e.g. not more than 3 mm. A further alternative is vacuum pyrolysis, in which heating rates may be relatively low, but the application of a vacuum quickly extracts the pyrolysis products and thus simulates some effects of fast pyrolysis.
Further, more recent, reviews of biomass pyrolysis have been conducted by A. V. Bridgwater (“Renewable fuels and chemicals by thermal processing of biomass” Chemical Engineering Journal Volume 91, Issues 2-3, 15 Mar. 2003, pages 87-102; and “Biomass fast pyrolysis”, Thermal Science Vol. 8 (2004), No. 2, pages 21-49).
It is known that the quality of bio-oil can be improved by the addition of ethanol. For example, in WO 2008/020167, it is disclosed that ethanol can act as a phase separation suppression agent for bio-oil. The problem addressed in WO 2008/020167 is that bio-oil tends to suffer from phase separation. Bio-oil is a complex mixture primarily of water, hydrophilic oxygenated organic compounds and higher molecular weight lignin fragments. It is considered that a water content above a certain level, e.g. 30-40% by mass can lead to phase separation of the bio-oil into two phases, an aqueous phase dominated by water and small hydrophilic organic compounds and an organic phase which contains most of the phenolic lignin-derived fragments. This phase separation is undesirable, since separate phases can be more difficult to handle and utilise than a single phase bio-oil.
Weerachanchai et al (2009) [P. Weerachanchai et al, “Phase behaviors and fuel properties of bio-oil-diesel-alcohol blends” World Academy of Science, Engineering and Technology 56 (2009) p. 387] disclose the results of work on attempts to blend slow pyrolysis palm kernel bio-oil with diesel fuel and alcohols. Slow pyrolysis liquids are invariably phase separated into an aqueous phase with soluble organic compounds such as acetic acid and an organic phase that contains a limited amount of water and water soluble organic compounds. The organic phase is usually higher viscosity and results from more extensive cracking of the pyrolysis products. Weerachanchai et al (2009) explains that it is known to mix diesel fuel with methanol and/or ethanol at up to 20% methanol/ethanol. In the experimental discussion in Weerachanchai et al (2009), it is explained that palm kernel bio-oil is obtained by slow pyrolysis of palm kernel cake at 700° C. The resultant bio-oil has two separated phases: an aqueous phase and an oily phase. The diesel used is a commercial petrodiesel fuel, i.e. a hydrocarbon. It was found that the solubility of petrodiesel in ethanol/bio-oil mixtures was only possible at room temperature up to a maximum of about 10% petrodiesel (in ethanol-rich ethanol/bio-oil mixtures). However, it will be appreciated that information about the miscibility of petrodiesel does not provide useful information about the corresponding miscibility of bio-diesel, in view of the very significant chemical differences between these compositions.
Garcia-Perez et al (2007) [M. Garcia-Perez et al “Production and Fuel Properties of Pine Chip Bio-oil/Biodiesel Blends” Energy & Fuels 2007, 21, 2363-2372] disclose the results of work on blending bio-oil with bio-diesel. The bio-oil was produced by slow pyrolysis of pine chips and pine pellets, resulting in a product with two separated phases: an oily bottom phase and an aqueous phase. Garcia-Perez et al (2007) disclose that the oily bottom phase is more soluble in bio-diesel than the aqueous phase. These workers aimed to check the solubility of the different bio-oil phases in bio-diesel by heating mixtures of bio-oil and bio-diesel to 60° C. and shaking. The mixtures were allowed to cool to room temperature. The oily bottom phase of the bio-oil mixed in a single phase with bio-diesel at up to 34 wt % of the oily bottom phase of the bio-oil. The aqueous phase of the bio-oil was essentially immiscible with bio-diesel. This disclosure demonstrates therefore that achieving any miscibility between all of the components of a bio-oil and bio-diesel poses a serious technical challenge.